The invention relates to a method for continuous monitoring of the quality of digital sound on distribution.
The digitalaudio coding processes used by radio- or TV-broadcasting services have made it possible to reduce the quantity of data to be transmitted. However, this reduction is liable to entail an irremediable loss of the quality of the sound by comparison with the original source signal.
The extent of the defects engendered depends simultaneously on the throughput allocated for the coder, on the complexity of the content of the sound signal, as well as on problems relating to the transmission of the signal.
For technical reasons or reasons of broadcasting responsibility, it is necessary to evaluate the quality level of the audio signal continuously. Subjective methods for evaluation of equipment, by human assessment and surveillance, are cumbersome to implement, and scarcely reliable. In particular, among the more specific drawbacks of the processes or methods of the prior art, mention may be made of:
the implementation of lengthy and expensive subjective evaluations;
the lack of completeness of the information which is necessary to carry out the monitoring of the perceived sound quality, when this information is supplied by the binary-stream analyzers;
the lack of objective analysis of the sound content, which alone is capable of reflecting the final quality of the perceived sound signals;
the defects inherent in differential analysis, such as:
making available the noncoded source, as a reference source;
sequences analyzed being of short duration, 20 seconds at most, which are not representative of the service analyzed;
transparency of certain defects to this type of analysis;
analysis being generally discontinuous, and not completely meaningful.
In particular, the processes of differential analysis, which are based on the human hearing system, between a reference sound source and the sound source to be evaluated, may allow automatic implementation. However, this solution appears to be impractical since it is necessary to have the reference sound source available.
The object of the present invention is to remedy the abovementioned drawbacks of the processes or methods of the prior art, by the implementation of a method based on a close study of the digital signal and of the continuous behavior thereof, so as, on the basis of conventional methods, to make it possible to assess the overall quality level of the signal.
The methods for continuous monitoring of the quality of sound on distribution, which is the object of the present invention, this digital sound being available in stereophonic mode with a digital signal representing at least one right-hand channel and one left-hand channel, consists in carrying out a statistical analysis of the content of this digital signal on each of these channels. The statistical analysis consists in segmenting the digital signal in the time domain into successive series of samples, including a defined number of samples, and, when a program of digital sounds is present, carrying out a spectral analysis of each of the series of samples in order to observe the variations in energy and in envelope of the digital signal in the time and frequency domains, and to calculate an overall quality index. A final quality index is calculated on the basis of the variations in energy and in envelope and of the overall quality index, in the form of a bounded value which is continuous in time, this final quality index being representative of the quality of the digital sound perceived.
The method, which is the subject of the present invention, finds an application to the operational and continuous surveillance of the sound components of audio and audiovisual services, before and after secondary distribution especially, to services for inspection of equipment, coders and multiplexers, for inspection of the quality of service, experimental platforms.